CoursesPhysiologyMuscular systemSkeletal muscleSkeletal muscle structure

Skeletal muscle structure

Learning objectives

After completing this study unit, you will be able to:

Describe the hierarchical structural organization of skeletal muscle from muscle to myofilament
Describe the relationship between T-tubules, sarcoplasmic reticulum and myofibrils
Explain the types, structure and composition of myofilaments
Describe the changes in a sarcomere during muscle contraction and relaxation using the sliding filament model

Introduction

Skeletal muscle is an organ consisting of skeletal muscle tissue, connective tissue and a rich neurovascular supply. It is surrounded by an epimysium, and is divided into bundles of muscle fascicles by the perimysium. Fascicles are formed by groups of individual muscle fibers, each surrounded by an endomysium.

Muscle fibers are muscle cells with a sarcolemma (plasma membrane) and sarcoplasm (cytoplasm). They contain contractile organelles known as myofibrils, sarcoplasmic reticulum (modified smooth endoplasmic reticulum), mitochondria, myoglobin and glycogen granules.

Myofibrils contain myofilaments, structurally organized to form repeating units known as sarcomeres, the functional units of contraction. Myofilaments include:

Thin filaments: actin, troponin and tropomyosin

Thick filaments: myosin

Elastic filaments: titin

Contractile (actin, myosin), regulatory (troponin, tropomyosin) and structural (titin) proteins play different roles in muscle contraction. The arrangement of filaments creates alternating dark and light striations on muscle fibers (A band, I band, H zone, M line).

According to the sliding filament model of muscle contraction, thin filaments slide over thick filaments to shorten the sarcomere, and generate muscle tension.

Explore concepts

Organization of skeletal muscle

Skeletal muscle has a hierarchical organization including muscle fascicles, fibers, fibrils and filaments.

Muscle Fiber

Muscle fibers are cells with specialized organelles that play an important role in muscle contraction and relaxation.

Myofibrils

Myofibrils are the contractile organelles of muscle fibers. Each myofibril is made up of myofilaments in a distinct organization pattern that gives skeletal muscle its striations.

Sliding filament model

The sliding filament model explains the process of muscle contraction, where thin filaments slide over thick filaments.

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Summary

Key facts about skeletal muscle structure
Organization	Skeletal muscle (organ) Skeletal muscle tissue: fascicles, fibers, fibrils, filaments Connective tissue: epimysium, perimysium, endomysium Nerves Blood vessels
Muscle fiber	Same as muscle cell Sarcolemma: plasma membrane Sarcoplasm: cytoplasm Sarcoplasmic reticulum: modified smooth endoplasmic reticulum (calcium storage) T-tubules: dips of the sarcolemma (conduct electrical impulses) Myofibrils: contractile organelle Mitochondria: ATP synthesis Myoglobin: oxygen storage Glycogen granules: energy storage
Myofibril	Contains myofilaments Thin filaments: actin, troponin, tropomyosin Thick filaments: myosin Elastic filaments: titin Muscle Proteins Contractile proteins: actin, myosin Regulatory proteins: troponin, tropomyosin Structural proteins: titin, nebulin Sarcomere: region between two Z discs
Striations	A band: dark band, thin and thick filaments I band: light band, thin filaments H zone: lighter portion of A band, thick filaments M line: dark line in H zone, anchors thick filaments Z disc: middle of I band, anchors thin filament
Sliding filament model	Thin filaments slide over thick filaments Myofilament length remains constant A band and M line: no change I band and H zone: reduced width Z discs: move closer together Sarcomere: shortens